The present invention relates to a semiconductor light emitting device.
In recent years, srmironrlrtcor light emitting devices have been widely used for information indication panels, optical communications and the like. These semiconductor light emitting devices are not only required to have high light emitting-efficiency, but also a high-speed response when they are used for optical communications in particular. Therefore, speed semiconductor light emitting devices having higher efficiency and quicker response are being vigorously developed.
A surface emitting type LED (Light Emitting Diode) allowing low current operation is drawing attention as a semiconductor light emitting device having high efficiency. However, the surface emitting type LED is relatively insufficient in high-speed response. The data transmission speed of the surface emitting type LED is at best around 100 Mbps to 200 Mbps. Consequently, a resonant cavity type LED has been developed. In the resonant cavity type LED, a light emission layer is located on an antinode of a standing wave formed by a resonator composed of two mirrors. Thereby, spontaneously emitted light is controlled to achieve high-speed response and high efficiency of the light emitting element (refer to Japanese Patent Laid-Open Publication HEI No. 3-229480 and U.S. Pat. No. 5,226,053). Recently, POF (Plastic Optical Fiber) has been used for high-speed communication system conforming to IEEE1394, USB2 and the like. As a preferable light source of POF, there has been developed a resonant cavity type LED in which AlGaInP based semiconductor material is used for its light emission layer. This LED enables high-efficient light emittance at a wavelength of 650 nm that is included in a low loss wavelength range in the POF (High Brightness Visible Resonant Cavity Light Emitting Diode: IEEE PHOTONICS TECHNOLOGY LETTERS VOL.10 NO.12 DECEMBER 1998).
However, the resonant cavity type LED having the AlGaInP based light emission layer has a problem in moisture resistance. The problem is caused by layers of AlAs and AlGaAs whose Al mix crystal ratio is close to 1 in the vicinity of the LED surface, as a result of using a multilayered reflection film made of AlGaAs based material as a mirror for forming a resonator. Also, the above-stated resonant cavity type LED has another problem that an optical output saturates when a current flow of several tens mA or more is applied. This problem is caused by insufficient diffusion of current because current applied from a surface of the LED diffuses only in DBR (Distributed Bragg Reflector) having approx. 1 xcexcm thickness. For solving these problems, there has been proposed an idea of forming a surface electrode into the shape of a honeycomb or mesh with a width of several xcexcm. However, such electrode has a drawback that breakage of the electrode is easily generated, which causes degraded reliability of LED.
Under these circumstances, there has been proposed a semiconductor light emitting device whose multilayered reflection film is composed of AlGaInP based materials, where the multilayered reflection film is formed on the surface side of the semiconductor light emitting device to constitute a resonator (refer to Japanese Patent Laid-Open Publication No. 2001-68732). This semiconductor light emitting device not only increases moisture resistance owing to the AlGaInP based materials which the multilayered reflection film of the device is made of, but also increases density of current applied onto a light emission layer owing to a current constriction layer. Further, a current diffusion layer is provided to solve the problem of optical output saturation.
In the above-stated conventional semiconductor light emitting device, however, current diffuses toward outside of a current flow pass formed by the current constriction layer. Thereby, the diffused current causes a low-current-density region to generate in the light emission layer. The low-current-density region is low in response speed, which causes a problem that the response speed of the entire device is decreased.
An object of the invention is to provide a semiconductor light emitting device having good high-speed response.
To achieve the object, the present invention provides a semiconductor light emitting device comprising:
a semiconductor substrate;
a first multilayered reflection film on the semiconductor substrate;
a light emission layer on the first multilayered reflection film;
a second multilayered reflection film made of AlyGazIn1-y-zP (Oxe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) on the light emission layer;
a semiconductor layer on the second multilayered reflection film; and
a current constriction layer on the semiconductor layer,
wherein the first multilayered reflection film and the second multilayered reflection film form a resonator with a specified interval, and the light emission layer is formed in a position of an antinode of a standing wave inside the resonator, and
wherein the semiconductor layer has a value obtained by dividing resistivity by thickness being 1xc3x97103 xcexa9 or more.
Since the semiconductor layer has relatively large resistivity and relatively small carrier density, a current in a current flow pass formed in the current constriction layer will not easily diffuse to the outside of the current flow pass. Therefore, there is generated few region with low current density in the light emission layer, which effectively improves a response characteristic of the semiconductor light emitting device.
Herein, when the semiconductor layer between the second multilayered reflection film and the current constriction layer has a value smaller than 1xc3x97103 xcexa9 obtained by dividing resistivity by thickness, an amount of current diffused to the outside of the current flow pass formed by the current constriction layer becomes so large as to pose bad influence to the response characteristic of the semiconductor light emitting device.
Conventionally, in the surface emitting type semiconductor light emitting device without resonator structure, a layer between the light emission layer and the current constriction layer was set to have a carrier density of approx. 3xc3x971018 cmxe2x88x923 for reducing series resistance. In such semiconductor light emitting device, it was not recognized that the carrier density of the layer between the light emission layer and the current constriction layer hindered increase of response speed. An inventor of the present invention has found out that the cause of current diffusion from the current flow pass formed in the current constriction layer correlates with the resistivity of the layer between the multilayered reflection film formed on the light emission layer and the current constriction layer, and has invented the present invention based thereon.
It is noted that throughout the specification, y and z in semiconductor compounds are independent in each semiconductor compound.
The present invention also provides a semiconductor light emitting device comprising:
a semiconductor substrate;
a first multilayered reflection film on the semiconductor substrate;
a light emission layer on the first multilayered reflection film;
a second multilayered reflection film made of AlyGazIn1-y-zP (Oxe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) on the light emission layer; and
a current constriction layer on the second multilayered reflection film,
wherein the first multilayered reflection film and the second multilayered reflection film form a resonator with a specified interval, and the light emission layer is formed in a position of an antinode of a standing wave inside the resonator, and
wherein a percentage of a current diffused to an outside of a current flow pass formed in the current constriction layer is 25% or less of a total current applied to the current flow pass.
Since the current diffused to the outside of the current flow pass is 25% or less of the total current applied to the current flow pass, it is possible to reduce the region with low current density in the light emission layer to the extent that bad influence is not given to the response characteristic of the entire semiconductor light emitting device. On the contrary, if the percentage of the current diffused to the outside of the current flow pass formed in the current constriction layer is beyond 25% of the total applied current, the regions with low current density due to the current diffused to the outside of the current flow pass become excessive, which deteriorates the response characteristic of the entire semiconductor light emitting device.
In one embodiment of the present invention, the semiconductor light emitting device further comprises a current diffusion layer on the current constriction layer.
Acccrding to the above embodiment, the current applied from the surface of the semiconductor light emitting device is uniformly led by the current diffusion layer to the current flow pass formed in the current constriction layer. Therefore, it becomes possible to effectively reduce operating voltage of the semiconductor light emitting device.
In one embodiment of the present invention, the light emission layer is made of AlyGa2In1-y-zP (Oxe2x89xa6yxe2x89xa61, 0 xe2x89xa6zxe2x89xa61). According to the above embodiment, it becomes possible to obtain emitted light at a wavelength range from 550 nm to 680 nm.
In one embodiment of the pregsent invention, a semiconductor layer made of AlyGazIn1-y-zP (Oxe2x89xa6yxe2x89xa61, 0xe2x89xa6z xe2x89xa61) is provided between the second multilayered reflection film and the current constriction layer.
Therefore, the semiconductor layer becomes transparent against the light with a wavelength of 550 nm or more, which enables highly effective extraction of emitted light with a wavelength of 550 nm or more.
In one embodiment of the present invention, a semiconductor layer made of GaP is provided between the second multilayered reflection film and the current constriction layer. The surface of this semiconductor layer is hardly oxidized, which makes it possible to grow a semiconductor layer with good crystallinity on top of this layer. As a result, it becomes possible to obtain a semiconductor light emitting device with less lattice mismatch and crystal defect and with good characteristics.
In one embodiment of the present invention, the current constriction layer is made of AlyGazIn1-y-zP (Oxe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61). Consequently, the current constriction layer becomes transparent against the light with a wavelength of 550 nm or more, which enables highly effective extraction of emitted light with a wavelength of 550 nm or more.
In one embodiment of the present invention, the current constriction layer is made of GaP. The surface of this semiconductor layer ishardly oxidized, which makes it possible to grow a semiconductor layer with good crystallinity on top of this layer. As a result, it becomes possible to obtain a semiconductor light emitting device with less lattice mismatch and crystal defect and with good light-emitting characteristics.
In one embodiment of the present invention, the current diffusion layer is made of AyGazIn1-y-zP (Oxe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61). Consequently, the current diffusion layer becomes transparent against the light with a wavelength of 550 nm or more, which enables highly effective extraction of emitted light with a wavelength of 550 nm or more.
Also, the semiconductor substrate is preferably made from GaAs in consideration of crystallinity of a semiconductor layer formed thereon.